1. Field of the Invention
The present invention relates to a method for determining operation modes and a related circuit, and more particularly, to a method for determining operation modes and a related circuit used in a power supply.
2. Description of the Prior Art
In order to decrease the package size, the pin count of the integrated circuit (IC) is also preferably smaller. The concept of a multi-function pin is thus developed. The power management IC of the power supply also needs the multi-function pin to reduce the packaging cost.
For instance, a light emitting diode (LED) driver IC, HV9910B, manufactured and sold by a company, Supertex, located in California, U.S.A., has a multi-function pin RT. FIGS. 1 and 2 show two light emitting diode (LED) driver systems, of which integrated circuit IC1 is HV9910B, suggested in the datasheet of HV9910B. The detailed system circuit operation of FIGS. 1 and 2 can be found in the datasheet of HV9910B. Briefly speaking, as power switch Q1 is turned on, inductor L starts to store the energy and current ILED flows from the power source VIN through LEDs LED1 to LEDN, inductor L, power switch Q1, and resistor RCS. If current ILED is as large as a preset value, then power switch Q1 is turned off, inductor L starts to release the stored energy and current ILED flows through a loop composed of LEDs LED1 to LEDN, inductor L, and diode D. As for the turned-off power switch Q1, it is again turned on depending on the operation mode. FIG. 1 shows the system operated in the pulse width modulation (PWM) mode; FIG. 2 shows the system operated in the constant off-time mode.
The difference between FIG. 1 and FIG. 2 is only the way for connecting resistor RT. If integrated circuit IC1 determines that one end of resistor RT is connected to the ground (GND) through multi-function pin RT, as shown in FIG. 1, integrated circuit IC1 would make the entire LED driver system operate in the PWM mode, such that switching frequency fPWM would be about a fixed value. If integrated circuit IC1 determines that the end of resistor RT is connected to pin GATE to receive a high voltage (e.g. 12V) therefrom, as shown in FIG. 2, integrated circuit IC1 would make the entire LED driver system operate in the constant off-time mode, such that off-time TOFF is about a fixed value. The resistance of resistor RT is also used to determine switching frequency fPWM in the PWM mode or off-time TOFF in the constant off-time mode.
In other words, multi-function pin RT is used not only to determine the operation mode of integrated circuit IC1 but also to determine the off-time of power switch Q1.
However, such multi-function pin RT design may cause the negative effects to the internal circuit design of integrated circuit IC1.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.